Note that punctuation of direct speech also depends on whether you're writing for the US market or not.
The US style is to always put the full stop or comma inside the quotation marks.
The non-US style is to use the logic of the text to decide the placement.
US:
“It is only a theory." said Frank.
“It is," said Frank, "only a theory."
non-US:
“It is only a theory." said Frank.
“It is", said Frank, "only a theory."
Note that the 'said' isn't capitalised just because the direct speech itself is a whole sentence. There's no need to change the full-stop to a comma.
http://www.oxforddictionaries.com/wo...-direct-speech fives a good summary.